Changes in Marker Frequencies. Changes in gene frequencies at the marker locus reflect the ability to capitalize on opportunities for selection on the marker. In addition to accuracy and other technical specifics of the genetic test, marker frequency changes depend on the ability to effect selection on the marker in the breeding program. Changes in marker frequencies can be readily evaluated for direct and LD markers based on population estimates and have been documented for some cases, in particular for genetic defects. For example, for bovine leukocyte adhesion deficiency in Holstein dairy cattle, the development of a genetic test (Shuster et al.,
1992) led to rapid elimination of carriers in U.S. bull studs, from over 150 in 1988 to less than five in 1992 (K. Weigel, University of Wisconsin, Madison, personal communication). Other examples of documented successes at this level are commercial lines that are specifically
marketed based on fixation of a particular gene or marker, for example the RYR and the RN genes in pigs